Xylan represents the major hemicellulose component in tropical plant biomass and world wide interest on microbial degradation of xylan has led to the discovery of several xylanases from a wide variety of microorganisms. The xylanase in all the instances studied are secreted in large quantities into the medium. In majority of cases the xylanases are also co-secreted with cellulase activity. The xylanases are by and large endoxylanases which result in the formation of xylo-oligomers leading to xylobiose as the predominant end product. The xylobiose is eventually hydrolyzed by B-xylosidase to yield the monomeric sugar xylose. Xylose finds extensive application as a source of the low calorie sweetener xylitol. The majority of the xylanases are co-secreted along with cellulasos and have their pH optima at or around neutral pH or even slightly acidic pH.
Recent interest on xylanases which are cellulase-free and stable at high alkaline pH has emanated from the realization that such xylanases could be extremely useful in the pulp and paper industries for biopulping, biobleaching and also in the manufacture of dissolving pulp. Besides being specific in their reactions at ambient temperatures, the use of xylanase also minimizes the use of toxic chemicals such as chlorine and chlorine dioxide which are environmentally hazardous (Viikari et al, In ACS Symp. Ser. 460: G. E. Leatham & E. M. Himmel (Eds.) American Chemical Society, Washington D.C.,1990). Strains producing alkaline xylanases are preferred to acidic xylanases for their use in the pulp and paper biotechnology. The search for cellulase-free alkali stable and active xylanases of microbial origin has resulted in the recent past in the discovery of several xylanases from bacteria, especially the genus Bacillus (Balkrishnan et.al. world J. Microbiol. Biotech. 8:626-31, 1992 Nakumura et. al. World J. of Mocrobiol. Biotech. 9:221-24, 1993) and actinomycetes (Vyas et. al. Biotech. Letters 12:225-228, 1990). Alkaline xylanases from bacteria are produced on either xylan or xylan-rich agricultural residues such as wheat bran at pH of 9.0 and above. These alkaline xylanases are active in the range of pH 7 to 9 and at temperatures between 50.degree. and 60.degree. C.
Fungi On the other hand are known to utilise biomass more efficiently and to secrete enzymes in larger quantities Fungal xylanases reported to date are active in the acidic range and are mostly co-secreted with cellulase.